Whitespace-mode

whitespace-mode is a minor mode to visualize blanks (tabs, spaces, newline, etc).

whitespace

Description	visualizes white spaces in the buffer
Author	name of author
Maintainer	FSF
Source	http://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/whitespace.el
Part of Emacs	yes

Basic setup

Whitespace-mode is already bundled with Emacs.
Use M-x whitespace-mode to activate it on the current buffer.

or add the following in your .emacs

(whitespace-mode 1)